Question

a) Describe how to prepare a 0.50 L of 0.10 M imidazole buffer, pH 7.50, starting with imidazole hydrochloride. Would you use NaCl or HCl to bring the pH to 7.50? b) Starting with imidazole, would you need NaOH to bring the pH to 7.50?

Answer 1

Using,

pH = pKa + log([base]/[acid])

7.50 = 7.11 + lof([Im]/[ImHCl])

[Im] = 2.45[ImHCl]

[Im] + [ImHCl] = 0.1 M x 0.5 L = 0.05 mols

2.45[ImHCl] + [ImHCl] = 0.05

[ImHCl] = 0.0145 mols

mass of ImHCl needed for buffer preparation = 0.0145 mol x 104.54 g/mol = 1.51 g

[Im] = 0.05 - 0.0145 = 0.0355 mols

So, moles of NaOH required to be added = 0.0355 mols

mass of Imidazole needed for buffer preparation = 0.0355 mol x 68.077 g/mol = 2.42 g

(b) Starting with imidazole (a base) we need to add HCl to prepare the given buffera and to bring the pH to 7.50